Although research on the bone morphogenetic proteins (BMPs) has expanded exponentially over the past decade, we actually know very little about the physiological roles that individual BMPs play in the skeleton. Our decision to focus this proposal on role the BMP3 in bone is based on several important findings. First, although BMP3 is the most abundant BMP in bone, accounting for 65% of the total BMP protein stored in bone matrix, little is known about its biology. A second reason for our interest in BMP3 are recent reports that correlate changes in BMP3 levels in vivo with fracture healing, mechanical loading of the skeleton, and the loss of endogenous bone formation with age. Finally, and perhaps most intriguing is our observation that BMP3 knockout mice display an unequivocal bone phenotype without a change in skeletal patterning. At five weeks of age, the total trabecular bone mass of BMP3 knockout animals is twice that of their wild-type littermates, demonstrating that BMP3 is a negative regulator of bone mass. In vitro, BMP3 is able to exert an inhibitory effect on osteoblast differentiation in the presence of potent osteogenic stimuli. The signaling pathway used by BMP3 to negatively regulate bone mass has not yet been identified. As osteogenic BMPs are now therapeutic agents used to augment bone formation in thousands of patients each year, understanding how BMP3 exerts its regulatory effects on bone mass has wide ranging clinical significance. In this proposal, we will: investigate what the lack of BMP3 signaling does to the formation, growth and maintenance of the skeleton (SA1); examine the effects of increased BMP3 expression on the formation, growth and maintenance of the skeleton (SA2); dissect the BMP3 signaling pathway (SA3), and define the function of BMP3 in regulating bone formation in the periosteum and bone collar (SA4). It is our belief that the information we obtain as a result of our experimental plan will be key to understanding how individual BMPs, alone and in combination with one another, function in skeletal patterning, bone formation, and the maintenance of bone mass.